The present invention is directed generally to permanent magnet assemblies and more particularly to permanent magnet assemblies for MRI systems.
There are various magnetic imaging systems which utilize permanent magnets. These systems include magnetic resonance imaging (MRI), magnetic resonance therapy (MRT) and nuclear magnetic resonance (NMR) systems. MRI systems are used to image a portion of a patient's body. MRT systems are generally smaller and are used to monitor the placement of a surgical instrument inside the patient's body. NMR systems are used to detect a signal from a material being imaged to determine the composition of the material.
These systems often utilize two or more permanent magnets directly attached to a support, frequently called a yoke. An imaging volume is providing between the magnets. A person or material is placed into an imaging volume and an image or signal is detected and then processed by a processor, such as a computer. The magnets are sometimes arranged in an assembly 1 of concentric rings of permanent magnet material, as shown in FIG. 1. For example, there may be two rings 3, 5 separated by a ring of non-magnetic material 7 in the gap between the magnet rings 3, 5. The ring of non-magnetic material 7 extends all the way through the magnet assembly 1 parallel to the direction of the magnetic field. The assembly 1 also contains a hole 9 adapted to receive a bolt which will fasten the assembly 1 to the yoke.
The prior art imaging systems also contains pole pieces and gradient coils adjacent to the imaging surface of the permanent magnets facing the imaging volume. The pole pieces are required to shape the magnetic field and to decrease or eliminate undesirable eddy currents which are created in the yoke and the imaging surface of the permanent magnets.
However, the pole pieces also interfere with the magnetic field generated by the permanent magnets. Thus, the pole pieces decrease the magnitude of the magnetic field generated by the permanent magnets that reaches the imaging volume. Thus, a larger amount of permanent magnets are required to generate a magnetic field of an acceptable strength in the imaging volume, especially in an MRI system, due to the presence of the pole pieces. The larger amount of the permanent magnets increases the cost of the magnets and increases the complexity of manufacture of the imaging systems, since the larger magnets are bulky and heavy.